This invention relates generally to devices for insufflating gas into a mass of molten metal such as steel, and more specifically, to an optical wear indicator that indicates when the device should be replaced.
The making of steel or other metals typically involves the introduction of gases into the ladle or vessel holding the molten metal to stir it. The gas is typically introduced into the ladle via a device called a stir plug. Such a stir plug may be mounted in the bottom or side of the vessel. Prior art stir plugs have taken numerous forms and constructions.
For example, one common type of stir plug comprises a solid, non-gas-permeable, conical refractory member disposed within a loose fitting metal or ceramic shell or canister. Such a "canistered" plug is commonly disposed within a seating block in the wall, e.g., the bottom, of the vessel holding the molten metal, and the purging gas is transported through the gap between the refractory cone and the metal canister into the molten metal.
Any refractory material stir plug device is subject to wear due to extreme operating conditions. As the stir plug is worn down, the longitudinal height of the plug decreases. Stir plugs must be replaced as soon as a certain critical minimal or remnant height is reached. If the stir plug is permitted to erode too much before it is replaced, a burn-out of the ladle in which the stir plug is located might occur, which is not only dangerous, but costly to replace.
Stir plugs with various devices to facilitate the determination of the critical remnant height have been generally available, but with certain tradeoffs. For example, electrical indicators are generally available, but they may be expensive to install and maintain, they require an external recording apparatus and are a possible source of disturbance in an already error sensitive system.
One type of electrical indicator is the device described in the U.S. Pat. No. 4,481,809 (LaBate) which utilizes several Hall effect transducers and circuitry to monitor the output thereof.
Another electrical indicator is that disclosed in German Patent No. DE 3,424,466 (Grabner) which utilizes two electrical wires within a probe. Both ends of the wires at the tip of the probe are separated. The wires consist of an alloy that melts at the critical temperature which indicates the critical wear height. The melting alloy closes the circuit and allows an electrical current to flow from the power source.
Temperature indicators are also available, but suffer from similar drawbacks as do the electrical indicators. One such device is the one disclosed in German Patent No. DE 3,526,391 (Fischer) which utilizes a thermocouple located inside the body of a ceramic stir plug. The critical temperature inside the plug is measured to determine the critical wear height of the plug. One deficiency of this device, however, is that in the event the temperature of the liquid metal destroys the thermocouple, the temperature can no longer be measured and therefore the plug may have to be prematurely replaced.
Other prior art devices measure thermal conductivity to provide an indication of the lifetime of a stir plug. For example, German Patent No. DE 3,833,503 (Rothfuss) discloses a valve configuration inside a ceramic gas stir plug. A low melting alloy keeps the gas flow valve control in the open position. Concurrent with the erosion of the stir plug, high temperatures will ultimately cause the alloy component to melt. This causes the gas flow valve to close, thus either reducing or eliminating gas flow. The reduced or discontinued gas flow indicates the stir plug wear.
Another device disclosed in German Patent No. DE 3,623,609 (Rothfuss) uses a gas flow restriction as a wear indicator. However, a gas flow restriction is not an unmistakable criterion, because a premature steel penetration of the gas passageways result in a low gas flow, thus causing a premature replacement of the stir plug.
German Patent No. DE 3,802,657 (Winkelmann) discloses a refractory wear indicator incorporated in a gas stir plug. This device is an optical indicator which makes use of the geometrical arrangement of the gas passageways situated inside the plug. A certain configuration of gas passageways inside the refractory cone separates the inner refractory portion from the outer one. The inner portion includes a round cross section at the tip of the stir cone. The round cross section changes into a square one at the bottom of the refractory cone. In use, when the metal has been tapped from the ladle and the stir plug is hooked up to a natural gas purging line, the natural gas escapes in a circle configuration from the plug. If the plug is worn down below the critical height, the configuration of the natural gas flames changes from round to square. This indicator system can only work as long as the gas passageways are free from steel. Steel penetration of the passageways prevents gas flow, thus rendering the passageways invisible.
U.S. Pat. No. 4,744,544 (LaBate et al.) describes a visual wear indicator for a metallurgical vessel that uses a metal rod which is inserted in the upper portion of a refractory body and extends inwardly of the surface of the body, at a length less than the known thickness of the refractory body. In this device, the metal rod and the refractory material therearound are elevated to the same temperature by the molten metal, but their light emission coefficients will be different, whereupon the end of the rod will glow red hot while the surrounding refractory material exhibits a different color (appearance). Thus, one can determine if the cone has worn down beyond the length of the rod. While this wear indicator is generally suitable for its intended purposes, it still leaves much to be desired. In this regard, since the refractory plug and the steel rod of the LaBate et al. patent have different thermal expansion coefficients, elevated temperatures will result in different expansions of both materials, which action may crack the refractory body. Moreover, a faster wear of the refractory body may occur, in part, caused by the drilling of the hole to accommodate the metal indicator. Further, the metal rod may be blown out of its hole by high gas pressure or it may prematurely melt away, thus effecting a premature plug exchange.
In U.S. patent application Ser. No. 08/059,403, filed on May 11, 1993, entitled Gas Stir Plug Wear Indicator Including Low Melting Point Component there is disclosed an claimed a stirring plug which provides more resistance to cracking and degradation than the LaBate et al. U.S. Pat. No. 4,744,544. That gas stirring plug comprises a shell, a core, and a low melting point component disposed within a cavity in the core. The core is located within the shell and is comprised of a refractory material extending between the bottom end and the top end of the plug. The cavity is of a predetermined height and has a bottom located a first predetermined distance above the bottom end of the plug. The cavity is filled with the low melting point component, e.g., soapstone, calcium silicate, talcum, or some other material having a melting point lower than steel, from its bottom to an intermediate point located a predetermined height above the bottom of the cavity. Mortar or some other high melting point refractory component is disposed in the cavity from the intermediate point to the top of the plug. The plug is operative so that when it has worn away to the intermediate point, e.g., the remnant height point, the low melting point component will be exposed to the molten metal, whereupon it will melt and flow out the cavity and molten metal and/or slag will flow therein to provide a predetermined visual appearance, e.g., a glowing dot surrounded by a darker area. That predetermined visual appearance changes, e.g., the glowing dot disappears, when the plug has worn away to the bottom of the cavity, thereby indicating that it should be replaced.
While the stirring plug of the patent application Ser. No. 08/059,403 provides more resistance to cracking and plug degradation than the aforementioned LaBate et al. patent, it never the less still leaves something to be desired from those standpoints. In this regard that stirring plug may also be susceptible to premature cracking under some conditions after the low melting point material has melted away and has been replaced by the metal and/or slag, due to the differences in the thermal expansion coefficients of the metal and/or slag and the refractory material making up the plug.
In German Offenlegungsschrift DE 3142989 there is disclosed several embodiments of a stirring plug formed of a porous refractory material body including a visual wear indicator embedded at the bottom of the refractory body. The visual wear indicator is formed of a refractory material having different light emission characteristic than that of the porous body so that when the plug has worn to its remnant height the wear indicator is exposed. The difference in light emission from the body and the wear indicator provides an indication of when to replace the plug.
In U.S. patent application Ser. No. 07/868,598 filed on Apr. 14, 1992, and entitled Gas Stir Plug With Visual Wear Indicator, there is disclosed a stir plug with a visual wear indicator which overcomes some disadvantages of prior art stir plugs. The stir plug of that application basically comprises a frusto-conical shaped member for introducing gas into a mass of molten metal. The plug has an outer core or body formed of a first refractory material and is located within a metal shell. Plural gas carrying slots may be provided in the body. A wear indicator in the form of a central core comprised of a second refractory material of higher porosity than the refractory material of the body is located within a centrally located recess in the body adjacent the bottom end of the plug. The central core extends from the bottom end of the plug towards the top end of the plug and is of a predetermined height less than that of the outer core. The upper end of the central core when exposed by the erosion of the stirring plug enables gas to flow therethrough so that it is cooler than the body, whereupon its light emission characteristic will be different than that of the body to provide a visual indication that the plug should be replaced.
While the stirring plug of the patent application Ser. No. 07/868,598 is generally suitable for its intended purposes since the wear indicator is located at the bottom of the plug and is of higher porosity than the refractory material making up the plug if the plug is left in operation after it has eroded substantially below the remnant height molten metal breach through the wear indicator.